1. Field of the Invention
This invention relates to a semiconductor device such as a memory, and more particularly to a semiconductor device which can easily and reliably carry out a conduction test.
2. Description of the Prior Art
A semiconductor memory is known as a typical example of semiconductor devices.
Generally, a semiconductor memory includes power supply terminals Vcc and Vss, an input signal terminal, an output signal terminal and an internal logic circuit. FIG. 7 of the accompanying drawings illustrates a general example. In this drawing, reference numeral 10 denotes a chip of a semiconductor memory. A logic circuit 5 is formed on this chip. A high voltage power supply Vcc terminal 1 and a low voltage power supply Vss terminal 4 are also disposed on the chip. Reference numerals 2 and 3 denote an input signal terminal and an output signal terminal, respectively. In this instance, a memory constituting an internal circuit outputs predetermined data to the output signal terminal 3 in accordance with the input signal applied to the input signal terminal 2 at a power supply voltage applied between the terminals 1 and 4 from outside.
Recently, test time has drastically increased in tests of semiconductor devices with the increase in memory capacity, and a test cost will become very high unless counter-measures are taken. To restrict the rise of the test cost, it is very important to discriminate as rapidly as possible rejected products. Furthermore, it is necessary to limit the increase of the test time by incorporating various specific test functions into the semiconductor devices so as to limit the rise of the test time.
The test items can be broadly classified into a DC test for checking a power supply current and an AC test for checking an access time, and the like. However, these performance guarantee tests are carried out on the assumption that all the terminals of the semiconductor devices to be tested (hereinafter referred to as the "devices") are in advance confirmed to have been reliably connected to the test terminals of a testing apparatus.
Unless the condition wherein all the terminals of the device are connected to the testing apparatus is guaranteed by this conduction test, performance of the device cannot be guaranteed even when the performance guarantee test is carried out. Conventionally, the following method has generally been employed in order to confirm that all the terminals of the device are reliably connected to the testing apparatus.
A diffusion layer made of an impurity having an opposite conductivity type to that of a substrate is connected to each terminal, and a diode is connected electrically. Therefore, the connection between the testing apparatus and the terminals of the device can be confirmed by forwardly biasing this diode. This test is repeated for each terminal of the device.
Speaking more definitely, various pins formed outside the device and connected to a pad portion of the device or in other words, its terminal portion, through leads, are brought into mechanical contact with a probe to be connected to these pins. Then, whether or not each pin is in complete contact with the probe is sequentially checked by use of a switching means disposed in the testing apparatus such as relay, and after all the pins are confirmed to be in perfect contact with the probe, the performance test of the device is commenced.
The time required for conducting such a conduction test has not been so long in the past because the number of terminals is not so great. Since the number of terminals has drastically increased in semi-conductor devices with the increase in their capacity in recent years, however, the time necessary for the conduction test has become longer and longer and has resulted in the increase of the test cost. Therefore, it has become very important to shorten the test time.
In other words, in the conduction test described above, the conduction test time increases in proportion to the number of terminals, and the test time will be ever-increasing unless measures are taken.
Therefore, a testing method which can efficiently conduct this conduction test has become essentially necessary.